Lets say, I could do it with the flat fins and specially with a longer fin. Fluttering your feet like a swimmer. Force Fins no.
Scooping all the way. I hung on to my FF for those real bad days and the hunkering down, but have not needed then. Able to move maybe a little faster specially on 4' swells with the longer fins (and oars). But, that is me.

So if I am understanding correctly with the Outcast or Force fins (rigid and not rigid) you only get propulsion on the up stroke with your leg?
Only the long scuba fins give you propulsion in both directions, up and down stroke?

Click to expand...

Simple answer: Regardless of fin, you'll always get more propulsion on the up stroke.

Rambling babbling answer...

- The amount of water that a fin can push in the exact opposite direction of the path of the tube relates to how efficient the fin is. A 100% rigid fin can for the most part only push water perpendicular to the blade at any given moment. Sure, water will spill off the end (and the sides) to some extent, but let's ignore that for now. To make a fine like this work (and I've proven it _doesn't_ in developing my UL fins!) you need to toe point down towards the bottom and push that back - breaking your ankle. Any time you kick up in a normal stroke with your foot level, the water gets pushed up and not back and this is wasted energy that gets you nowhere. Same on the down stroke. For the most part, you'll get more thrust on the up kick because as you kick up you also kick out which moves more water back. On the down kick you are kicking in (in the direction of movement), which moves less water. This is regardless of the type of fin. I think the Force Fins are optimized to maximize the upkick thrust due to the unique channel. The up-angle of the Force Fins may help allow you to walk, but in my opinion it works against efficient float tube propulsion because you have to put more toe point down in to create the backwards thrust vs. upwards "churn". The channeled shape, though, probably does make it easier to kick down so you don't waste energy on that stroke (when it doesn't create as efficient thrust anyway).
- As the fin flexibility is increased, the amount of toe point down required is decreased because when the fin flexes it vectors the water thrust relative to perpendicular all along the fin (it's more complicated than just this but it should help understand what's going on). Even if you keep your foot level on this stroke, the fin tip and much of the rest of the blade are angled down (on the upstroke) and therefore pushing water back. The reduced toe point down helps reduce or eliminate those terrible cramps as well.
- There is such a thing as too flexible however so you have to dial in the rigidity to minimize toe point down and resulting cramping, but still be rigid enough to move enough water back to allow for sufficient "torque" to kick upwind and move at a practical clip otherwise. Also, too big a blade or too long a blade, even if it makes a nice curve in the water and vectors water efficiently, can have so much resistance that cramping and fatigue will still be an issue.

If I were to redesign Force Fins for float tube use (they were originally scuba fins so they are probably optimized for swimming and they happen to work for float tubes) I would eliminate the upward angle and maybe lengthen the fin a couple of inches and make them a bit more flexible. Also I'd make them float!

One other design compromise of the Force Fins' center fold and upward angle, while making the down kick easier, reduces their ability to kick you FORWARD. Now, moving forward in a float tube is clunky, and mostly only good for positioning in zero wind, but it is useful in certain circumstances. Most flat blade fins can be made to move you forward as you kick under the seat but it's never elegant.

If I were to redesign the Outcast backpacking fins (flat plastic with strappy ankle pocket) I'd make the plastic more flexible overall, especially towards the ends.

One last variable that will change everything is how much of your leg is in the water. Back in the day with donut tubes you were submerged up to your waist or deeper so your kick was a lot like a person in scuba doing a backstroke. Now with inflatable seats and pontoon boats, the amount of water you have available to kick up is reduced so you can't maximize this motion and your fin eventually breaks the surface - this is especially true with pontoon boats so your efficiency takes a dump. Still, I wouldn't trade only having my legs below my knees submerged for anything!

Maybe I should consider doing a float tube fin "Kickstarter"? No pun intended!

I do like the way the foot pocket is designed on the standard Force Fins. The foot hoop is butted up to the top of your foot at the ankle and not further toward your toes which helps reduce cramping. The worst thing in the world in fins is using your toes to fight the water resistance. Cramp city!

Shig, thank you for your technical description! All of it is on the mark. I have used just about everything that has been talked over...except Omegas. For me, the Force Fin "adjustables" were a pain: they would loosen up, and way too heavy while wearing boots. Now the standard Force Fins are great: I bought a used pair of XXL and 2mm flats booties. My feet slide way into the 'strap' and I don't get cramps, good propulsion and my feet stay much warmer. I have scuba dived since the early 70's and don't want to think how muchI have spent on the newest and greatest fins! I know I won't ever buy another pair unless I get to give them a 'Test Drive'.